Heavy metals, such as lead and cadmium, are presented in high amounts in rest-products (coke, ashes, slag) from waste-incineration. In contact with water the heavy metals leak into the surroundings. Heavy metals such as lead and cadmium are extremely toxic and poisonous, and are known to cause human cancer and effect reproductivity. In Denmark, the amount of lead that is collected for deposition and incineration is 1400-3400 tons/year (Miljxc3x8styrelsen, 1997). After incineration around 90% of the heavy metals are found in the ashes, coke and slag. The increased usage of PVC-material has lead to tons of waste, which is usually disposed off by incineration or deposited at controlled waste disposal sites. Upon incineration of PVC material, ashes and coke are formed with a high content of lead and chlorine. PVC alone contribute to ⅔ of the total amount of chlorine which is incinerated. In fact, the presence of chlorine increases the leakage of heavy metals to the surroundings. Increased amounts of waste for deposition or incineration will lead to an increased pollution caused by the heavy metals and chlorine.
The international patent application WO 9716230 relates to separation of metal chlorine compounds from metallurgical dust. The metallurgical dust is washed to remove the metal chlorine compounds, followed by extracting with nitric acid to dissolve lead, zinc, cadmium, copper, magnesium, calcium and manganese. The pH is raised to 1.8-3.5 to precipitate iron. Finally lead, copper and cadmium is removed. The filtrate is evaporated and decomposed to obtain solid metal oxides and calcium nitrate. Zinc is recovered by further processing.
DE 4217133 relates to a process for removal of chlorine and heavy metals from dust and slag. The process is based on washing the material with an organic acid, filtration of the mixture and separate disposal of filtrate and filter residue. 70% of the lead is separated from the dust and slag.
In another patent application (EP 551155) heavy metals such as lead, zinc, copper and cadmium are extracted and recovered from flue dusts. This is done by treating the flue dusts with an aqueous solution of ammonium and sodium chloride. Lead is in this case dissolved as a chloro complex. The resulting solution is separated from the solid residue. Finally the mixture is further treated to separate zinc.
EP patent no. 482335 describes a process for removal of heavy metals from fly ash particles. This is done by washing with an acidic solution, rinsing and disposing the residue. The acid washings are then treated with for example Ca(OH)2 to give a heavy metal filter cake.
U.S. Pat. No. 5,102,556 proposes a process for removal of heavy metals from iron. based sludges. The sludge is dissolved in hydrochloric acid and is filtered to remove lead chloride.
None of those known processes separate in a simple and economic way the waste into pure materials and/or materials which can be disposed in the nature without problems.
These problems are solved by the present invention.
The present invention proposes a new technology for separation of heavy metals and halogens from coke, ashes, fly ash, slag, metallurgical dust and the like waste products. By exploiting the influence of pH on the metal solubility in water, metals and halogens can selectively be extracted from the waste. The separation sequence include multiple extraction and filtration stages. By treating a waste, for example coke from a PVC recycling process, lead can be separated from the coke into a lead product of a 50-60% lead purity, which can be further concentrated. The chlorine can be separated into a salt with less than 100 ppm lead content. Finally, the coke product has been raffinated for more than 98% of the initial chlorine content, and 90% of the initial lead content.
The process of the invention also propose to separate heavy metals (such as lead) by extracting in an acidic aqueous solution (nitric acid) like some of the known processes as well as use filtration and precipitation as separation techniques. As a new feature, the process of the invention use changes in pH to dissolve, precipitate and separate lead and chlorine from coke.
The process of the invention is a process for separation of heavy metals and halogen from unwanted waste material or residues containing a mixture of these materials, wherein
a) the waste material is optionally comminuted,
b) carbondioxide optionally is removed from the optionally comminuted material,
c) the halogen is selectively extracted or washed out from the optionally comminuted and optionally for carbondioxide removed material providing an essentially halogen free waste material (A) containing heavy metal, and a liquid (B) containing the halogen,
d) the metal is selectively extracted or washed out from the waste material (A) from step c) at a low pH providing a waste material essential free from heavy metal and halogen,
e) the steps c) and d) are optionally repeated, and
f) the extracted metal or metals is optionally precipitated.
The unwanted waste material can for example be a residue as produced by a PVC recycling process as described in PCT/DK96/00117 or from an incineration plant. This residue will have to be milled or in another way comminuted, whereas other waste materials need not be comminuted.
The heavy metal can be metals such lead, copper, zinc, cadmium, chromium, tin, manganese and nickel and mixtures of two or more metals.
The halogen can be chlorine, bromine or fluorine or a mixture of two or all three. The halogen is bound halogen, normally bound as an inorganic compound.
Steps c) and d) can be combined into one step.
Steps c) and d) can also be repeated as b),b),b . . . c),c),c) . . . as many times as wanted using pure water or recycled washing liquids from former steps, and water in the last washing step.
The process of the invention can thus be performed with or without recycling of the filtrates, in order to minimise the consumption of chemicals.
A basic pH in step c) is achieved by using a base such as Al2OH3, an alkaline or earth alkaline metal hydroxide or a mixture thereof. Specific examples of usable bases are NaOH, CaOH2, KOH, MgOH2. CaOH2 is preferred, because it leads to an end product useful as a fertiliser, as thaw salt or industrially useful in another way.
An acidic pH in step d) is achieved by using an acid. The acid can be selected from the group consisting of HCl, HNO3 and acetic acid. HCl is preferred. By using CaOH2 and HCl for treating the coke from a PVC recycling process according to for example PCT/DK 9,600,117 it is possible to convert the waste material totally in reusable materials. The lead can be separated from the coke into a lead product of a 50 to 60% lead purity, which is such a high concentration, that it is economically feasible to recover the lead therefrom. The chlorine is separated as CaCl2 with a lead content of less than 100 ppm, which permits use of this material without polluting nature. Finally, the coke product has been raffinated for more than 98% of the initial chlorine content and 90% of the initial lead content.
Step c) can be conducted at any pH , but pH 7-11 is preferred. Even more preferred is pH 9-10.
Step d) can be conducted at pH 0-4, preferably pH 1-2.
In the present invention is has been shown, that the liquid-solid ratio is important. In both step c) and step d) it is possible to use a ratio of 0.5:1 to 50:1. In step c) is it preferred to use a ratio of 1.5:1 to 3:1 and in step d) a ratio of 3:1 to 5:1.
It is possible to use a temperature of 0 to 200xc2x0 C. in step c) as well as in step d). Higher temperatures will often give a faster process, but at a higher cost. Step c) is therefore normally performed at room temperature. Step d) is usually performed at 20 to 100xc2x0 C.
Extracted metal from step d) is preferably precipitated at a high pH, but it is also possible to recover the metal in another way, for example by electrolysis of the solution.
It is possible to use atmospheric pressure or a higher pressure.